Abstract: For a high-speed motor unit, it is very important to accurately predict dynamic characteristics of the rotor for high speed performance at the design stage so as to minimize the likelihood of failure. Finite element (FE) analysis and experimental measurements are used to calculate the critical speeds, and the modes of multi-span shafting coupled by flexible coupling, and the axial, radial and torsional stiffness of elastic coupling are simulated using the spring element of FE analysis tools. The influence of coupling stiffness and leading design parameters of coupling are analyzed. The results show that the critical speeds and mode shapes of shafting are different from a single rotor, and can be adjusted by changing the stiffness and structure of coupling in order that the shafting has sound dynamic properties.